It is sought more particularly here below in this document to describe problems existing in the field of geophysical data acquisition for analysing the sea-bed (e.g. for oil exploration industry using seismic method). The invention of course is not limited to this particular field of application but is of interest for any technique for localizing a marine animal in a marine environment that has to cope with closely related or similar issues and problems.
Regulation agencies, such as JNCC (“Joint Nature Conservation Committee”), MMS (“Minerals Management Service”), IBAMA (“Brazilian Institute of Environment and Renewable Natural Resources”), DFO (“Department of Fisheries and Oceans”), which have as objective the protection of marine mammals, encourage or impose the use of a PAM system during seismic survey campaigns.
These regulating agencies propose guidelines defining rules to apply during seismic campaigns in order to protect the marine mammals. In particular the guidelines recommend the PAM system to detect the presence of marine mammals in the vicinity of seismic sources comprised in the PAM system, which sources can be considered as injurious to the marine mammal life (e.g. acoustic disturbances). A safety exclusion zone is generally defined by the regulation agencies around the seismic sources (e.g. the JNCC agency defines a regulated exclusion zone of 500 meters) to exclude the presence of the marine mammals or limit time of exposure to acoustic waves produced by the seismic sources.
In the prior art, a PAM system typically comprises a network of acoustic sensors, such as hydrophones or geophones or accelerometers or vector sensors, arranged along one or several linear acoustic antennas (hereafter referred to “streamers”) and adapted for detecting and measuring vocalizes, i.e. acoustic signals emitted by marine mammals. When a marine mammal vocalizes in the vicinity of the network of seismic sensors, these sensors make measurements of the acoustic signal or signals emitted by the marine mammal. A localization computation is then performed from the measurement data collected by the sensors to determine the localization of the marine mammal from the seismic sources.
There are two known families of PAM systems used by the oil prospecting companies:                the integrated PAM systems, which rely on the use of a network of existing seismic sensors distributed on the streamers towed by the seismic vessel, which seismic sensors being originally adapted to perform geophysics data acquisition,        the independent PAM systems, which rely on the use of a network of dedicated seismic sensors arranged along a linear acoustic line, which is deployed in addition to the existing streamers behind the seismic vessel and entirely dedicated to the passive acoustic monitoring (hereafter referred to “dedicated PAM line”).        
The prior art PAM systems inform the operators of the marine mammal positions over the survey, which positions are generally split into a range and a bearing referred to the seismic vessel. The positions are then compared to the safety exclusion zone and when a marine mammal is detected inside the safety exclusion zone, then constraints on the seismic survey operations are imposed, such as the shutdown of the seismic sources or the change of some seismic source characteristics (e.g. amplitude or frequency acoustic signals emitted by the seismic sources).
Despite the fact that the errors of the locations can be significant compared to the dimensions of the safety exclusion zone, the prior art PAM systems currently allow carrying out a localization computation, but do not give any uncertainty on the mammal position, to the operator. As a consequence, the final decision taken by the operator (for example to turn off the seismic sources) is taken without knowing the real risk of presence of the marine mammal inside or outside the safety exclusion zone.
FIG. 1 illustrates, in a functional block diagram, a simplified example of a classic configuration of a PAM system implementing a known method for localizing a marine mammal.
On detection of vocalizations, the seismic sensors 1 (hydrophones), 2 (vector sensors), 3 (accelerometers) of the PAM system carry out acoustic signal measurements (scalar absolute pressure for the hydrophones 1, non-scalar acoustic sound velocity for the vector sensors 2, non-scalar sensor acceleration for the accelerometer 3), which are then collected by a signal processing unit 4. After processing, the unit signal processing unit 4 generates time difference of arrival (TDOA) data 10 and angle-of-arrival (AoA) data 11 to a computation unit 5, which is adapted to provide a value of marine mammal localization 15 by applying a known localization method, such as least-square or beamforming or triangulation or a cross-bearing. Data of position of the seismic sensors (arrow 12) and data of underwater acoustic sound celerity (arrow 13) are also provided to the computation unit 5. The latter can also take into account a hypothesis on the depth at which the marine mammal is estimated to be (arrow 14).
Thus, to summarize, whatever the known localization method used by the PAM system, the marine mammal localization by a PAM system requires processing the following set of measured data:                data derived from the acoustic sensors measurements:                    time difference of arrival (TDOA) data;            angle-of-arrival (AoA) data;                        data of position of the seismic sensors (which can be measured data or predefined data),        data of underwater acoustic sound celerity (which can be measured data or predefined data).        
TDOA data can be computed thanks to measurements of time-of-arrival (TOA) of a marine mammal vocalization or measurements of difference of phase between two seismic sensors receiving a vocalization of a marine mammal. A measurement of angle of arrival of the marine mammal vocalization can be carried out either between two seismic sensors (e.g. between two synchronized hydrophones) or on distinct seismic sensors (e.g. an acoustic vector sensor or an accelerometer).
Theses known localization methods may also need to do some assumptions, namely:                an assumption on the marine mammal depth can be made if the PAM system does not allow to get enough data to completely solve the localization problem,        an assumption on the underwater acoustic sound celerity assuming that acoustic waves are propagating in straight line.        
However all measured data used by the known localization methods are generally affected by errors. It therefore leads to an uncertainty on the marine mammal localization:                Sensor position data, TDOA data, AoA data, and acoustic sound celerity data are generally affected by errors, thereby introducing a significant error in the localization computation. Moreover, wrong hypothesis made on the sound celerity and/or the marine mammal depth and/or on the position of seismic sensors, also contribute in increasing the uncertainty of localization of the marine mammal.        The seismic sensors positioning is an issue for a network of seismic sensors towed on streamers or a dedicated PAM line, as the movement of the seismic vessel towing the sensor networks as well as the hydrodynamic instabilities at the sensors induce feather on the network of sensors. Moreover, the towed sensor network often lacks the possibility to measure in real time the real position of the sensors in space. Then, the sensor locations used by the localization method are very rough estimates and generally distort the localization performances of the marine mammal.        The accuracies of TDOA and AoA data have also a significant impact on the localization uncertainty. The TDOA and AoA data are computed by signal processing methods which are applied on measurement signals acquired by the different seismic sensors. The accuracies of TDOA and AoA data depend on:                    the marine mammal species (vocalization emission level, vocalization beam pattern, vocalization frequency bandwidth, vocalization duration),            the environment (ambient noise level, reflected paths, acoustic sound celerity profile, etc.),            the processing applied to the measurement signals (sampling frequency, processing gain, etc.).                        An error on the celerity measurements also introduces an error on the marine mammal localization computation. Some sound velocimeters are generally integrated to the streamers but only measure the celerity at the streamers depth, whereas the celerity at the marine mammal depth can be significantly different, in particular, in presence of thermocline effects.        One skilled in the art could easily compute this errors using algorithms such as the best standard deviation computation in order to provide an uncertainty area. However, predefining errors is complex and as the environment is constantly changing, predefined errors could be adapted for a short period of time but not the following.        